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Liu et al. Microstructures 2023;3:2023020 https://dx.doi.org/10.20517/microstructures.2023.02 Page 15 of 27

[57]
[60]
and copper in the solid solution state . In the soluble precipitates mechanism [Figure 6C] , the element
preferentially promotes the formation of soluble precipitates and dissolves, promoting pit nucleation. This
mechanism applies to copper after aging during which epsilon-Cu is formed. Epsilon-Cu corrodes and
[60]
initiates pitting . In the soluble inclusions mechanism [Figure 6D] , the alloying elements forms soluble
[72]
inclusions. The inclusions preferentially dissolve and nucleate pits. This mechanism applies to the addition
[65]
[72]
of sulfur . In the insoluble inclusions mechanism [Figure 6E] , the alloying elements form insoluble
inclusions, which do not corrode. Pitting corrosion occurs in the matrix near the inclusions. This
mechanism has been previously reported for titanium . The last mechanism is the wrapping mechanism
[65]
[Figure 6F] . The element forms a protective layer that wraps the soluble inclusions. When the inclusions
[66]
dissolve, the protective layer protects the matrix from pitting corrosion [66,69] .
Microstructure
Ferrite and austenite
The Volta potential difference between the ferrite and austenite phases is approximately 50-100 mV
[Figure 7A] [28,77] . The potentiodynamic results show that the dissolution potentials of austenite and ferrite
are -281 mV and -323 mV in 1 M H SO respectively . Theoretically, the potential difference should
[78]
2
SCE
4,
SCE
induce micro-galvanic corrosion and accelerate the corrosion of the ferrite . However, duplex stainless
[79]
steel has excellent corrosion resistance. Xiao proposed that there is feedback during the corrosion
process . The corrosion of the ferrite phase can change the passive film of the austenite phase. Xiao also
[80]
believed that the potential difference is no longer the main electrochemical inhomogeneity in corrosive
media, where both phases can be passivated . Tian et al. found that the passive current density of the
[80]
[51]
duplex stainless steel was lower than that of the single-phase structure . The two phases in the duplex
stainless steel may interact during the corrosion process. Cheng et al. found that the defect density in the
two-phase passive film is lower than that of single austenite phase or single ferrite phase . They proposed
[81]
that the two-phase galvanic corrosion effect improves the corrosion resistance of the duplex stainless steel.
However, the exact galvanic corrosion mechanism is unclear. Additionally, the existing test methods are
controversial. The single-phase structure obtained by anodic dissolution is honeycomb-like. Therefore, the
single-phase samples have more interfaces, which increases the risk of crevice corrosion of the samples
during electrochemical testing.

Pitting corrosion after deformation has also been studied [28,78] . Örnek and Engelberg reported that the ferrite
phase of 2,205 undergoes pitting corrosion preferentially before cold deformation, whereas the austenite
[77]
phase preferentially pits after a 40% cold deformation . Luo et al. also found that when the cold
deformation of UNS S31803 exceeds 70%, pitting corrosion preferentially initiates in the austenite phase .
[82]
Molyndal et al. proposed that the corrosion resistance of ferrite decreases monotonically with strain but not
in the austenite phase . Slip bands and subgrains are formed in austenite under a small deformation, which
[78]
barely deteriorate the pitting corrosion resistance. High-angle grain boundaries are formed in austenite
under large deformation, which deteriorates the pitting corrosion resistance . Therefore, the relative
[78]
pitting corrosion resistance between ferrite and austenite reverses as the cold deformation increases
[Figure 7B1 and B2] . However, the above studies were based on the correspondence between pitting
[83]
corrosion behavior and microstructural changes after deformation. Evidence for the direct correlation
between microstructural changes (dislocation configuration, slip banding, and subgrain boundaries) and
pitting corrosion is still lacking.

The phase ratio affects the pitting corrosion as well [Figure 7C1 and C2] . Ha et al. stated that the smaller
[23]
the PREN difference between the two phases, the stronger the pitting corrosion resistance of the entire
matrix [21,22] . This view was verified in 2,205 and 2,101 duplex stainless steels. However, the pitting corrosion
resistance of different phase ratios in 2,507 stainless steel The pitting corrosion resistance with different